Recent advances implicate error-prone DNA polymerases in the generation of virtually all mutations induced by environmental carcinogens in higher eukaryotic cells. These data have supported the promise of cancer chemoprevention based on the selective modulation of the activity of these proteins, based on the assumption that reducing the mutant frequency will reduce the incidence of cancer. However, carcinogenesis studies using newly-developed mouse models in which one or another of these polymerases is deficient have yielded unexpected results. Specifically, the deficiency of one such polymerase, termed DNA polymerase iota (pol iota), resulted in greatly decreased mutation frequencies induced by ultraviolet (UV) radiation in an endogenous reporter gene. These results would predict a protective effect against the carcinogenic effects of UV. However, pol iota-deficient animals exhibited a highly accelerated development of aggressive skin cancer. This unexpected result is not consistent with the somatic mutation hypothesis of carcinogenesis, and highlights the fact that there are critical gaps in our knowledge of the cellular function of this universe of polymerases. This application proposes to examine the putative tumor suppressor function of pol iota, and is directly responsive to the current Program Announcement since modulation of the activity of enzymes in this pathway has been proposed as a potential antimutator strategy. This application addresses the overall hypothesis that DNA polymerase iota acts as a tumor suppressor by a mechanism that is distinct from its activity as an error-prone polymerase. To examine this, we propose a pilot project that will consist of two Specific Aims. In Aim 1, we propose to examine the effect of polymerase iota deficiency on UV-induced changes in gene expression and on damage-induced cell cycle checkpoints in murine and human cells. In Aim 2, we will examine the effect of polymerase iota deficiency in UV carcinogenesis studies that employ novel murine models. This application will fill critical gaps in our knowledge of how cancer is initiated by the most ubiquitous environmental carcinogen. The ultimate goal of this research is to understand how carcinogens cause cancer in order to design strategies to prevent the disease.